The present disclosure generally relates to magnetic latching actuator for use within an electricity meter. More specifically, the present disclosure relates to electrical contactors that are utilized within a domestic electricity meter to selectively connect or disconnect the electricity mains to a home or business serviced through the electricity meter.
Domestic homes and small businesses receive electricity from a main through an electricity meter that includes circuitry for measuring the amount of electricity consumed by the home. Typically, the electricity meter includes two bus bars each having an infeed blade connected to the electricity mains and an outfeed blade connected to the wiring of the home. In electronic electricity meters, circuitry within the electricity meter measures the amount of electricity consumed, typically across two phases. In North America, for example, the two bus bars in an electricity meter provides phase voltages at approximately 115 volts to neutral for low power distributed sockets or 230 volts across both phases for high power appliances such as washing machines, dryers and air conditioners, representing load currents up to 200 amps.
In many currently available electronic electricity meters, such as the Icon® meter available from Sensus Metering Systems, the electricity meter includes a radio that can receive and transmit signals to and from locations remote to the meter. The ability of the electronic electricity meter to receive information from locations/devices remote to the meter allows the electronic electricity meter to perform a variety of functions, such as reporting electricity consumption and selectively disconnecting the home from the electrical mains. As an example, utility providers may require some homes to pre-pay for electricity. When the prepayment amount has been consumed, the utility may desire to disconnect the electricity mains from the consumer's home to prevent further electricity consumption. Alternatively, the utility may wish to disconnect the electrical mains to a home for any number of other reasons.
Many metering specifications demand that any component included within the meter that is subjected to excess overload current conditions, including power disconnect contactors, must be capable of surviving demanding overload criteria, especially when subjected to a range of potentially damaging short-circuit fault conditions. As an example, commonly utilized testing standards require the contactors within the meter to survive an overload condition thirty times the nominal current rating.
Contactors for domestic supply applications typically may have nominal current capacities of 200 amps. Under testing conditions, these contactors are expected to survive thirty times these nominal current values for six full supply cycles. This represents overload levels of 7,000 amps RMS or peak AC values of almost 12,000 amps.
Domestic metering power disconnect contactors have to survive this arduous overload current condition as described above. One of the issues created during the overload condition is the magnetic force created by the extremely high current values passing through the fixed feed blade and a moving contact blade during the excessive overload situation. If the contacts are arranged such that the direct current flow through the fixed and movable contacts is opposite each other, the magnetic forces may urge the contacts to separate. As an example, under standard load conditions, the magnetic force attempting to separate the contacts may be approximately 1 Newton. During overload test conditions, as many as several hundred Newtons may be acting to separate the contacts.
In such meter designs, the fixed and movable contacts are held in the closed position and moved from the closed to an open position by some type of actuator assembly. Such actuators must also be able to survive the arduous overload current conditions described during testing conditions and must hold the contact in the closed position during such testing conditions.
Another problem that exists in conventional remote disconnect switches within electricity meters is that the electrical contacts within the meter wear over the lifetime of the switch. In a 200 amp remote disconnect, where a typical contact opening distance is on the order of 2 millimeters, the wear over the lifetime of the contact components in the direction of closure can be on the order of 0.5 millimeters. This amount of wear represents a significant percentage of the overall movement of the contact.
In order to overcome this wear issue, many remote disconnect switches utilize a compliant member between the actuator and the moving contacts. This compliant member is frequently the bus bar to which the moving side of the contact pair is attached. This method of indirect application of force to the contact to achieve closure leaves the contact vulnerable to bounce, inconsistent closure force or flexing of the bus bar under high current, all of which cause increased wear and higher resistance or higher likelihood of failure.
A common actuator used for opening and closing contact pairs in commercially available remote disconnects is an electromagnetic solenoid. Electromagnetic solenoids are particularly suitable since they typically operate sufficiently quickly (within one line cycle) such that any arc struck between the contacts will extinguish at the next zero point crossing, rather than being maintained over a relatively long period. Electromagnetic solenoids used are usually bi-stable solenoids that latch at the end points of their travel by employing either mechanical or magnetic latching functions to hold the contactor state. The latching force is typically a steep function of position as the ends of the actuator travel are approached, as the reluctance drops rapidly as the moving iron parts close on the stationary iron parts, resulting in an increasing flux in the gap. The steep force curve results in the use of a compliant member described above positioned between the actuator and the moving contacts. Most compliant members have a resultant force that varies as the displacement varies. Some of these issues can be overcome by employing a constant force spring structure; however, these spring structures can be complex and have issues with dynamic response.
As described above, it is desirable to provide a combined actuator arrangement and electrical contactors within an electricity meter that allow the electricity meter to operate satisfactorily through testing conditions while also being able to separate the contacts within the electricity meter over an extended period of use.